From a mobile, in every case that I know of, calling numbers starting 01 02 or 03 will be cheaper than any number starting 08 (including "freephone" numbers, which mobiles generally charge extra for.  The only exceptions are for a very small numbers of helplines, mostly starting 0808 80, which will still be free.)

From landlines, it's somewhat more complicated.  Depending on your call package and provider, sometimes 0845 and 0870 calls might be inclusive, so incur you no charge, for certain times of day.  Outside these times, BT charge less than geographic rates for 0845 to some customers, wheras other providers charge more.

Also from BT, 0844/0843 calls cost up to 5p/min, which for some numbers may already be cheaper than landline calls, and for all ranges from 1st October, if you would be paying for calling landlines..

So, it's almost impossible to say which is the cheapest number to call in any given situation.  If you have a telephone package which gives inclusive calls or a minutes allowance these are generally limited to 01/02/03 numbers, so using a number from this range would be the best to use as you'd not be paying extra for this call.  Some landline providers have started adding 0845 and 0870 ranges only, which may therefore be callable at certain times.  If you are calling from a landline outside any inclusive call period or allowance, then it really depends on the provider and tariff which is best to use, and therefore only you can answer that.

Dave wrote on Sep 8^th, 2009 at 9:27am:

I agree with Dave, this is better than my attempts at explaining this concisely.

I do however have sympathy with Rattybag, who is NOT being thick. These issues do test the brain and demand far more time and energy than most people are able or ready to commit to perhaps saving a few coppers on a phone call. In some cases however it is more than a few coppers.

It is BT which has made this issue particularly confusing by charging less for calls that cost it more to place. It has succeeded in getting a few others to follow it. We also have the equivalent of the "fly to Paris for £1" providers in the market. Whilst these perverse prices are genuine we cannot oppose them, even though it makes the situation more complex than we would wish and appears to undermine the essential point that we are making.

Please bear with us Rattybag, we are doing the best that can be done.

irrelevant wrote on Sep 8^th, 2009 at 9:49pm:

Indeed it is.

Please all forgive me for going on about this, but it is made much simpler if you explain how revenue sharing causes a premium charge to be passed from the caller to the called, and then say "BT charges may vary".